1. Field of the Invention
The present invention relates to wireless communications and, more particularly, to a method for performing a handover procedure and creating data.
2. Discussion of the Related Art
A cellular scheme is a concept proposed to overcome the limitation of service area, and frequency and subscriber accommodation capacity. It provides a speech area (call area, or a communication range) by changing a single base station of a high output into a plurality of base stations of a low output. Namely, a mobile communication service area is divided into a plurality of small cells, and different frequencies are allocated to neighbor cells while the same frequency band is allocated to cells which are so away from each other that no interference is generated therebetween to thus spatially re-use frequencies.
Handover (or handoff) refers to a function according to which when a user equipment (UE) moves and is released from a current communication service base station (referred to as ‘serving cell’, hereinafter) and enters a neighbor cell of an adjacent communication service area, the UE is automatically tuned with a new traffic channel of the adjacent communication service area and maintains its call state. That is, when the UE is communicating with a particular base station and if a signal strength of the particular base station (referred to as ‘source base station, hereinafter) weakens, the UE is linked to a neighbor base station (referred to as ‘target base station, hereinafter). When handover is supported, a call interruption, that may be otherwise generated when the UE moves to a neighbor cell, can be resolved.
Handover may be defined as three separated steps, a handover preparation, a handover execution, and a handover completion.
The UE measures a signal transmitted from the serving cell, and if a channel quality is drops to below a certain threshold value, the UE transmits corresponding information to the source base station through a measurement report. Then, the source base station determines handover according to the measurement report. Upon determining handover, the source base station a handover request to the target base station and then receives a handover request ACK from the target base station. The source base station informs the UE about a start of handover by sending a handover command. The processes up till now belong to the handover preparation.
After the handover preparation, the UE moves to the target base station through the handover execution and the handover completion processes. At this time, the source base station stops downlink data transmission of a PDCP (Packet Data Convergence Protocol) layer and forwards downlink data to the target base station.
In this respect, however, an RLC layer and a PDCP layer of the UE in the downlink transmission and an RLC layer and a PDCP layer of the base station in the uplink transmission do not know the handover execution and completion processes. Accordingly, the RLC layers cannot help but transfer SDUs (Service Data Units) in sequence regardless of handover. This is the same even when the RLC layers cannot transfer the SDUs to the PDCP layers due to a blank of PDUs corresponding to some sequence numbers. The PDCP layers also cannot forward nor reorder downlink or uplink data.
Due to such a restriction, if handover is completed with a blank of a PDU, other PDUs following the blank PDU are all removed. The downlink data removed at the RLC layer of the UE can be restored as the PDCP layer of the UE requests re-transmission of the downlink data from the PDCP layer of the target base station after handover is completed. And the data removed at the RLC layer of the base station can be restored as the PDCP layer of the target base station requests re-transmission of the data after the handover is completed.
However, because such restoration requests restoration between the PDCP layer of the UE and that of the target base station, namely, requests the re-transmission in the wireless communication, radio resources are wasted, and because the restoration time is delayed, the transmission efficiency is degraded.
Thus, in order to solve the problem, a handover executing method and data creating method allowing data transmission/reception between the RLC layer and the PDCP layer while the handover is being performed is requested.